Water courses may be contaminated with substances such as sewage, heavy metals, pesticides containing organic residues etc. Such substances act as free radical scavengers. They can therefore be assayed using a light-producing free radical reaction; the light emission is reduced or inhibited to a degree proportional to the amount of contaminant present, when compared to a distilled water control.
One such test, available under the trade name Aquanox, from Randox Laboratories Limited, involves a free radical reaction between a hydrogen acceptor (oxidant) and a hydrogen donor (luminol) in the presence of an enhancer. This reaction is catalysed by horseradish peroxidase (HRP) and results in light emission at a constant rate.
At present, in order for the Aquanox assay to be performed, a vial of freeze-dried signal reagent (containing luminol, enhancer and oxidant) is reconstituted with 5 ml borate buffer (pH 8.5), and then 100 .mu.l of this solution, 20 .mu.l enzyme reagent plus 1 ml water sample are added to a disposable cuvette. The reaction is started by the addition of the enzyme reagent. The separate additions of signal reagent, enzyme and sample to the cuvette have proved difficult for some users when undertaken in the field, especially to less technically skilled personnel.
Sol-gels are known. The sol-gel process involves the mixing of metal alkoxides, e.g. TMOS, i.e. tetramethoxysilane, in solution with water and a catalyst, at room temperature. In this process, a complex series of hydrolysis and polymerisation reactions occurs. The initially fluid solution becomes viscous as polymerisation proceeds, until a gel is formed. The gel is then dried, during which process liquid is expelled, causing substantial volume shrinkage of the gel, leaving a dry porous solid. The pore networks formed in dried gels do not scatter visible radiation and are therefore optically transparent.
Molecules added in the sol-gel process become entrapped in the growing covalent network. For example, Piechota and Sueverkruep, Acta. Pharm. Technol. 34:27s (1988), discloses sol-gels of unspecified composition, containing fluorescein. The marker was released at an essentially constant rate over 5-8 hours, and the rate varied with increasing phosphate content.
Further, EP-A-0439318 discloses a reagent trapped in a sol-gel glass, with the intention that a desired reaction with an analyte should occur within the pores of the network. The Examples disclose TMOS:solvent ratios of approx. 2:3 for sol-gel shapes and 1:8 for thin films.